1. Field of the Invention
The present invention relates to an improved process for the liquefaction of raw subdivided coal. More particularly, the invention relates to an improved liquefaction process wherein the coal is dissolved in a hydrogen donor-type solvent, passed through a guard bed, and hydrocracked.
2. Prior Art
Coal is our most abundant indigenous fossil fuel resource, and as a result of dwindling petroleum reserves, concerted research efforts are being directed towards recovery of liquid hydrocarbons from coal on a commercial scale. A promising approach in this field relates to the direct liquefaction of coal accompanied with minimum gas production. This approach has principally evolved from the early work of F. Bergius, who discovered that transportation fuels could be produced by the high-pressure hydrogenation of a paste of coal, solvent and catalyst. Later discoveries revealed the advantageous use of specific hydrogenation solvents at lower temperatures and pressures. With these solvents, such as partially saturated polycyclic aromatics, hydrogen is transferred from the solvent to the coal molecules, thus causing depolymerization and dissolution of the coal. The resulting coal liquid, however, has a high molecular weight and a corollary high viscosity, which presents considerable obstacles in removing the fine coal residue particles remaining in the liquid, since these particles typically range in size from 1 to 25 microns in diameter. The complete nature of the coal residue, or undissolved solids, is not wholly understood; however, the residue appears to be a composite of organic and inorganic species. The residue organic matter is similar to coke, and the inorganic matter is representative of the well-known coal ash constituents. Removal of the residue from the coal liquid has been considered a critical step in the prior art in the preparation of clean fuels, particularly in those processes in which the coal liquids are subjected to catalytic upgrading, such as hydrocracking. In hydrocracking processes, the presence of the coal residue contributes to catalyst pore plugging, which results in prematurely shortened run times.